First-time pet owners are often in for a small shock the first time they see their dog actually play in the classroom. “OMG! He is growling, and that dog just bit his neck! That other dog over there is HUMPING another dog! Wait, my dog just bit the other dog in the neck, too! This is too aggressive. I’m uncomfortable.”

Let me get one thing straight: Dogs don’t “do” Legos, video games, or Barbies. They jump, chase, bite, growl, bark, and wrestle. Many of the things that happen in play may look like aggression, and it is true that play can escalate into aggression rather quickly if left unchecked.

So how can you tell when that display of shiny teeth means fun instead of business? Here are a few signs of healthy play to look for.

1. Reciprocity and role reversal

“I scratch your back, you scratch mine” is the mantra. I like to see frequent role reversal in play with friendly dogs — I chase you, you chase me. I tackle you, you tackle me. I bite your neck, you bite my neck.

If I consistently see one dog tackling another and the other dog is not offered the opportunity to tackle back, it is often more bullying than play, and the owner of the dog who is tapping out on the ground needs to step in.

When it comes to chasers, you have some dogs that only like chasing and do not like being chased. This can be a problem for the playmate, who says, “Hey, I want to chase, too!” If the chase-ee is always trying to get away from the chaser, you should definitely intervene.

2. Take frequent breathers

I also like to see frequent, self-imposed breaks. “Breathers” are just that — seconds-long pauses where each dog gathers her wits before re-engaging in play. Breathers don’t have to last very long, and then the dogs will reinitiate with one another.

If dogs do not give these frequent self-imposed breaks to one another, it is a good idea to step in and make them do so before releasing them back to play; just ask for a few simple behaviors and then release your dog to join the fun. I find that humans often have to do this when two friendly dogs first meet, but that the dogs quickly learn to regulate themselves after minimal intervention.

3. Happy dogs have Jell-O bones

“Jell-O-y” is my uber-scientific technical term for the body language of happy dogs. When dogs enter the dog park with stiff tails and stiff bodies, they are asking for trouble. If one dog looks like he has a vibrating coat hanger stretched straight up through his tail, the “play” interaction will likely not be very enjoyable for anyone.

There is a looseness in the body language of happily engaged dogs: Tails are swishy, tongues are lolling, bodies are wiggly, eyes are sparkling. Much as play is a topic worthy of its own book, so is canine body language. My friend Carol Byrnes, owner of Diamonds in the Ruff training center in Spokane, has two great CD-Roms on canine body language, What Is my Dog Saying? and What Is My Dog Saying at the Dog Park? The latter is a must-review if you want to learn more about the mechanisms of dog body language as it relates to canine social interactions.

4. Make sure your dog gets plenty of exercise

Every time I see a dog enter the dog park on a tight leash, eyes bulging, panting and out of control like he hasn’t had adequate exercise in ages, I cringe. If your dog comes in like a freight train, it’s not exactly safe for the other dogs who may be near the tracks!

5. Go with the flow

Dogs who are playing well are essentially going with the flow. You will want to see nearly constant, nearly fluid movement. Watch for changes or disruption in the flow — maybe a higher- or lower-pitched vocalization than during the previous play.

Similarly, there is a difference between a “breather” (where dogs typically have very soft eyes and open mouths, tongues lolling out) and a visual lock-on, where the dogs stop playing and one or both will get stiff and stare. This is a signal for you to step in and quickly diffuse tension by creating space for the dogs. If the “chase-ee” has previously been running with a happy, loose tail, but you see it go between her legs and tucked firmly against her belly, step in and help her before things go wrong.

6. Smart owners make happy dogs

By far the best sign that play will go appropriately is when you carefully select your dog’s playmates, teach yourself about play and body language, and know your dog well. Some dogs, as they get tired, become cranky and have less patience in play. Others take progressively more and longer breaks until they are sleeping.

Know your dog, know the dogs your dog is interacting with, and have a plan in place. What are potential conflict triggers for your dog? Do you have an intervention strategy planned, and if so, at what stage will you implement it?